Chemical mixture problems are a unique type of GMAT word problem. These problems typically give you the percentage breakdown of a certain volume, and ask you to add or subtract certain elements in order to obtain a new volume. The trick to solving chemical mixture problems is to properly organize the information given. A table is a great way to accomplish this. See the example below for details.

Example

Bob just filled his car’s gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?

1. 9/10
2. 1
3. 10/9
4. 20/19
5. 2

Solution

1. The question is asking for how many gallons of ethanol must be added to form a mixture of 10% ethanol and 90% gasoline
2. Let x = # gallons ethanol added
3. Translate the information provided into equation form.


In mixture problems, I like to use a table to keep information organized. This is a valuable tool for higher level difficulty problems. Allow one column to be the list of ingredients, the other column to be the actual volume. Always remember to double check units and to make sure they match!



Since we know the new mixture will consist of 10% ethanol, the new volume of ethanol divided by the new total volume should equal 10%.



4. Solve:

10/9 gallons of ethanol must be added to the mixture. The answer is C.

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